When ISL encounters an internal error, it sets the error flag, but it is
not isl_error_quota that was already checked. Check for general errors
and abort the schedule optimization if that happens, instead of
continuing on the good path.
The error occured when compiling llvm-test-suite's
MultiSource/Applications/JM/lencod/leaky_bucket.c with Polly enabled.
Not adding a test case because it depends on ISL internals. We do not
want to a test case to depend on which version of ISL is used.
305cf623d7